The rotary drilling process creates a bore hole in the earth by use of a drill bit which is attached to a drill stem. A drill stem consists of, from top to bottom, a kelly, drill pipe and drill collars. The drill bit and drill stem are lowered and rotated into the earth creating a bore hole by breaking, abrading and fracturing the earth beneath the drill bit. During this process drilling mud is circulated by means of a pump down the inside of the drill stem and up the annular space between the outside of the drill stem and the wall of the bore hole. The drilling mud removes the cuttings, cavings and other debris from the annular space. Cuttings are the chips of earth which are created by breaking, abrading and fracturing the earth beneath the drill bit. Cavings are the pieces of earth which have fallen or sloughed into the annular space from the bore hole wall. Other material such as oil, gas and water from the bore hole wall or from beneath the drill bit are also entrained in the drilling mud. Also, the contact between the bore hole wall and the drill stem and mechanical failure of the drill stem occasionally cause parts of the drill stem to be entrained in the drilling mud. The products of abrasion between casing and cement which holds the casing in place and the drill stem are occasionally entrained in the drilling mud.
Other functions of the drilling mud include cooling and lubricating the drill bit, and maintaining a hydrostatic pressure on the bore hole wall which is greater than the pressure in the earth. This hydrostatic pressure prevents uncontrolled flows of oil, gas and water from the earth into the bore hole.
Determining the composition of the earth at the site of the drill bit as it drills within the earth at the end of the drill stem is one of the major functions of the well site geologist. Typically, in the area where drilling occurs, layers of various rocks were deposited one on top of the other. These layers--or rock strata--are usually shale (a compacted form of clay or mud), sandstone (a compacted form of sand), limestone, dolomite, coal, salt, anhydride, and sylvite.
It is known that the task of identifying rock strata is aided by detecting the naturally occurring gamma radiation in the strata, which is produced (along with alpha and beta rays) when unstable radioactive elements in the rock strata decay into stable elements, thereby releasing gamma rays. After the well is drilled or during the critical phases of drilling, the gamma radiation can be measured by removing all the drill stem and the drill bit from the bore hole. Open hole tools are lowered on a cable to the bottom of the hole and then hoisted to the surface while recording the gamma radiation of the bore hole wall. A drawback of this technique is that well site personnel are required to wait until after the bore hole is drilled to a suitable depth and the drill stem and the drill bit are removed before gathering the bore hole gamma radiation data.
Another technique used to identify the composition of the earth at the site of the drill bit is to measure the drilling penetration rate to indicate hardness changes in the rock strata being drilled. However, the advent of new drilling bits which maintain a constant drilling penetration rate as the rock strata changes precludes the well site geologist from relying on this technique.
Another technique used to identify the composition of the earth at the site of the drill bit, which does not require removing the drill stem, is to visually examine the material entrained in the drilling mud circulated from the bore hole. This is costly, because it requires a geologist or other professional to be on-site during the drilling in order to maintain a detailed description of the entrained materials as they are circulated from the bore hole. Occasionally, rig personnel fail to collect the entrained material or collect it at the wrong time or mix up the samples. Also, modern drilling bits abrade the cuttings into very fine chips. Finely abraded chips are difficult to sample in the mud circulation system and are difficult to examine visually.
A difficulty with all techniques which analyze the material entrained in the drilling mud is that the physical characteristics, including the natural gamma radiation of the drilling mud, are constantly changing during the drilling operation. Drilling mud is generally a mixture of water, clay, barite and other chemicals which are designed to provide the properties which drilling personnel require for safe and economical drilling. Often drilling personnel use water for drilling and as the water is continuously circulated the clay or shale from the substrata is ground into fine particles which are suspended in the water. This drilling mud is called native clay drilling mud. Drilling personnel maintain and often change the characteristics of the drilling mud by adding water, drilling clay, barite (a material which increase the hydrostatic head) and other materials which alter the physical properties of the drilling mud or by using other drilling fluids. All these combinations are referred to as drilling mud herein. Occasionally the drilling mud is discarded so that other drilling mud can be introduced into the bore hole or the undesirable properties of the current drilling mud can be eliminated. These changes in the drilling mud have a dramatic effect on the gamma radiation readings measured in the mud return line. This is because the gamma radiation of the drilling mud is a high proportion of the total gamma radiation of the drilling mud combined with the entrained materials. For example, the clay used as a component of the drilling mud is bentonite that is generally three to four times more radioactive than the shales encountered while drilling and up to forty times more radioactive than the sandstone or limestone encountered while drilling. The drilling clay can be up to thirty percent of the drilling mud by volume, but typically is in the range of two to twelve percent by volume.
Other factors also vary the gamma radiation readings in the mud return lime. A large pumping rate for the drilling mud will result in larger gamma radiation readings in the mud return lime, since a larger volume per time is then flowing past the detector. Bentonite drilling mud varies in its pumping rate from two hundred to five hundred gallons per minute. Finally, the drilling rates typically encountered may vary between one half a minute per foot to five minutes per foot. A faster drilling rate will result in a larger gamma radiation reading than a slower drilling rate, since a larger volume of entrained materials is then moving past the detector. Drilling rates vary from area to area and tend to be slower at deeper depths.
As can be seen, the composition of the drilling mud, the flow rate, the drilling rate and the volume and type of entrained material in the mud return line are constantly changing. These changes affect the gamma radiation measurements in the mud return line.
Yet another technique for determining the earth composition at the drilling bit, which does not rely on an analysis of the materials entrained in the drilling mud, is measure while drilling (known as MWD) gamma ray logs. MWD logs utilize gamma ray instrumentation placed in the drill collars near the drill bit. The instrumentation records and telemetries the gamma ray measurements to the surface and the data is processed and can be presented as a graph of natural gamma radiation versus depth.
MWD logging is effective, but is not available in many wells due to cost and logistics. Moreover, MWD logging tells nothing about caving of the bore hole wall into the bore hole, since it only measures the gamma radiation at the drill bit. Therefore, a process is desired that will produce an accurate measure of the gamma radiation at the drilling bit and also measure the gamma radiation of any caving into the bore hole.